Sedimentary Rock Properties
Sedimentary rocks are made up of particles of different sizes, shapes, and compositions held together by a matrix material of cement or other finer particles. The nature of the particles and the matrix determines many of the important behaviors of the rock, such as strength, durability, and susceptibility to moisture. The Ferm classification focuses on the following properties for describing sedimentary lithologies.
Grain Size or Texture. Particle sizes range from clays (which are micron scale and visible as individual grains only under a microscope) to millimeter-scale sand grains (which can be seen with the eye) to centimeter-scale pebbles. The general size of grains can be determined visually and by feeling the grittiness or smoothness with fingers. For siliclastic rocks, grain size defines the major rock groups. Conglomerates are composed of sandstones with pebbles. Sandstones are composed of sand-sized grains. Siltstones are composed of silt-sized grains. Mudstones are fine-grained (silt- to clay-sized grains) unlaminated rocks including siltstones and claystones. Shales have similar grain size, but are laminated. Claystones are composed of clay-sized grains. Most limestones in coal-bearing rocks are fine-grained, but coarse varieties do occasionally occur.
Mineral Composition. The general composition of sand-sized and larger grains can be directly observed. The particles may be composed of common sedimentary minerals or other kinds of rock fragments. General mineral composition is used as part of the description of sandstones and conglomerates in the Ferm classification.
Color. The most obvious property of rocks, color is closely related to particle composition and serves as its proxy when grains are too small to see. Natural rock color is best observed in fresh, unweathered specimens. Weathering can change the color of a rock by altering its mineral content. In core, rocks may be weathered near the surface, and along fractures. Color is commonly used as a modifier in rock descriptions. In the Ferm classification, it is used in the description of fine-grained clastic rocks.
Fabric. This property describes how the grains are arranged within the rock at the scale of a hand specimen. Grains may be highly aligned on a regular or consistent plane (parallel to each other) or irregularly aligned.
Sedimentary Bedding and Structures. Most rocks included in the Ferm classification were deposited in water bodies such as streams, bays, lakes, or oceans. Currents responsible for transporting the grains result in a variety of structures that are diagnostic of the flow regime at the time of deposition. Bedding structures include commonly described rock laminations and bedding, such as ripples and crossbeds. Bedding structures are used in descriptions of both clastic rocks and carbonate rocks.
Biogenic Structures. After initial deposition, sediments may be modified by the action of plants or animals living in the environment. These activities result in distinctive structures and rock fabrics that are diagnostic of plant rooting or the burrowing activities of invertebrate animals (called bioturbation).
Deformation Structures. After initial deposition, sediments may also be modified by mass movements related to instabilities in the environment. These movements can result in changes to texture, fabric, or sedimentary structure. Examples of these structures are dewatering structures, flow rolls, and slumps.